In conventional arts, there has been proposed, for example, “an impeller of a centrifugal fan including a plurality of blades, a main plate to which the blades are fixed, and a shroud fixed to end faces of the blades on the side opposite to the main plate, wherein recesses arranged substantially parallel to the axis of rotation are provided in the whole or part of the suction surface of each of the blades, and the width and depth of the recesses gradually increase from the main plate side to the shroud side” (see, for example, Patent Literature 1).
There has been proposed, for example, “an impeller (42) of a centrifugal fan including a plurality of hollow blades (44) annularly disposed around the rotating shaft, each of the hollow blades including a first surface portion (51) integrally molded with or fixed to the main plate and made of resin, and a second surface portion (61) attached to the first surface portion to form a hollow space (S) between itself and the first surface portion and made of resin . . . wherein . . . the end face of the edge portion closest to the second surface portion is in contact with the end face of the second surface portion closest to the first surface portion and thereby forms a blade shape retaining mechanism for preventing the second surface portion from being deformed toward the outer peripheral side by centrifugal force” (see, for example, Patent Literature 2).